Synthesis and characterization of novel pyridine-isourea complexes of Pd(II)

Article abstract of DOI:10.1016/j.poly.2012.02.014

New neutral palladium(II) complexes having formula PdCl ₂(pyiu^Pr), PdCl₂(pyiu^Cy), Pd(OCOCH₃)₂(pyiu^Pr), Pd(OCOCH₃) ₂(pyiu^Cy), Pd(CH₃)Cl

Full text of DOI:10.1016/j.poly.2012.02.014

Polyhedron 37 (2012) 66–76
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of novel pyridine–isourea complexes of Pd(II)
a,
a
b
Marco Bortoluzzi a, , Gino Paolucci , Federica Sartor , Valerio Bertolasi
⇑
⇑
a Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Dorsoduro 2137, 30123 Venezia, Italy
b Dipartimento di Chimica e Centro di Strutturistica Diffrattometrica, Università di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy
a r t i c l e i n f o
a b s t r a c t
New neutral palladium(II) complexes having formula PdCl2(pyiuPr), PdCl2(pyiuCy), Pd(OCOCH3)2(pyiuPr),
Pd(OCOCH3)2(pyiuCy), Pd(CH3)Cl(pyiuPr) and Pd(CH3)2(pyiuPr), where pyiuPr and pyiuCy are the novel
ligands 1,3-diisopropyl-2-((pyridin-2-yl)methyl)isourea and 1,3-dicyclohexyl-2-((pyridin-2-yl)methyl)
isourea, were synthesized and characterized. These neutral ligands act as bidentate and coordinate the
palladium center through the pyridine N-atom and one of the nitrogen atoms of the isourea dent, as
deducted by variable-temperature NMR experiments and DFT calculations and observed by X-ray diffrac-
tion experiments on the complex PdCl2(pyiuCy).
Article history:
Received 25 January 2012
Accepted 13 February 2012
Available online 21 February 2012
Keywords:
Palladium
Isourea
Pyridine
Ó 2012 Elsevier Ltd. All rights reserved.
N-donor ligands
Seven-membered rings
1. Introduction
coordination sphere [5]. Finally, several complexes of late transi-
tion metals with isourea ligands having coordinating groups
Amidines and amidinate ions are well-known ligands in coordi-
nation and organometallic chemistry. These N-ligands are charac-
terized by several different possible coordination modes and
usually bind one or two metal centers. Their electronic and steric
features can be tuned on changing the groups bonded to the nitro-
gen atoms and to the carbon atom between the two nitrogen atoms
[1]. Moreover, ligands containing more than one amidinate group
have been synthesized [2] and polydentate ligands have been ob-
tained also through the introduction of suitable substituents on
the amidine C or N atoms [3].
Isoureas can be considered species analogous to amidines,
where an alkoxy group formally replaces the alkyl group or the
hydrogen atom bonded to the amidine carbon. Despite of this sim-
ilarity, the coordination chemistry of isoureas has been scarcely
investigated. Palladium(II) complexes with two coordinated al-
kyl-substituted isourea molecules have been obtained from the
reaction of methanol with Pd-coordinated carbodiimides or by
reacting Na2PdCl4 with two isourea equivalents. X-ray diffraction
studies have shown that the coordination of the N-donor ligands
in these complexes occurs through the imine nitrogen atom [4].
A Pd(II) isourea complex bonded through the imine N-atom and
a C-atom of a phenyl ring has been reported by Vicente et al. This
species has been obtained by reacting dicyclohexylcarbodiimide
with a palladium precursor containing a C-bonded phenol in the
bonded to one of the nitrogen atoms have been published. Exam-
ples of these additional coordinating fragments are the benzoyl,
the ethoxycarbonyl and the carbamimidoyl groups [6].
The few data actually reported in the literature do not allow a
comparison of the electronic and steric features of alkyl-substituted
isourea ligands with respect to other, more common N-donor li-
gands. Our research group is currently interested in the synthesis,
characterization and reactivity of palladium(II) complexes with
polydentate nitrogen-donor ligands [7]. To the best of our knowl-
edge, isourea groups with alkyl substituents on the nitrogen atoms
have not yet considered as potential dents for the preparation of
new chelating ancillary ligands. Square-planar palladium(II) com-
plexes containing novel bidentate N-donor ligands are of para-
mount interest in several fields of applied coordination chemistry,
as an example as catalytic precursors for olefin polymerization
and co-polymerization [8]. In this paper we report the synthesis
of the pyridine–isourea ligands 1,3-dicyclohexyl-2-((pyridin-2-
yl)methyl)isourea (pyiuCy) and 1,3-diisopropyl-2-((pyridin-2-yl)
methyl)isourea (pyiuPr). The coordination chemistry of these N-do-
nor ligands towards the Pd(II) metal center has been investigated
by NMR and X-ray diffraction techniques.
2. Experimental
2.1. Materials and methods
⇑
Corresponding authors. Tel.: +39 0412348651; fax: +39 0412348517
(M. Bortoluzzi), tel.: +39 0412348563; fax: +39 0412348517 (G. Paolucci).
All synthetic work was carried out under an inert atmosphere
using standard Schlenk techniques. The reagents 2-pyridinemetha-
nol, N,N0-dicyclohexylcarbodiimide, N,N0-diisopropylcarbodiimide,
E-mail addresses:
(G. Paolucci).
markos@unive.it
(M. Bortoluzzi),
paolucci@unive.it
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2012.02.014

M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
67
1,5-cyclooctadiene (COD), tetramethyltin, potassium tert-butoxide,
methanesulfonic acid, palladium(II) chloride and palladium(II) ace-
tate were purchased from Aldrich and used without further purifi-
cations. Copper(I) iodide was an Acros product. Solvents were
purified following common procedures [9]. The NMR solvents were
purchased from Euriso-Top, with the exception of deuterated nitro-
methane (Aldrich). The complexes cis-PdCl2(NCCH3)2, PdCl2(COD)
and Pd(CH3)Cl(COD) were synthesized following reported proce-
dures [10].
Elemental analyses (C, H, N, Cl) were carried out at the ‘‘Istituto
di Chimica Inorganica e delle Superfici’’, C.N.R., Padua. The purifica-
tion of the stable products before the elemental analyses was car-
ried out by slowly cooling (from 30 to ꢀ25 °C during a week) clear
solutions of the ligands and the complexes dissolved in mixtures of
solvents such as dimethylformamide, nitromethane, acetone,
dichloromethane and diethylether. The vials or glass tubes con-
taining the solutions were cooled in a jacketed glass vessel con-
stirring at room temperature for 24 h. The solvent was then re-
moved by evaporation under reduced pressure, obtaining a dark
brown oil which was purified by chromatography on silica gel,
using ethyl acetate as eluent. After elimination of the solvent by
evaporation under reduced pressure, pentane (20 mL) was added
and the resulting solution was filtered. Removing the solvent under
vacuum isolated the final product. These purification operations
must be carried out quickly to avoid the formation of decomposi-
tion products and the products must be stored at low temperature.
The products start decomposing after few days also if stored at
ꢀ20 °C under inert atmosphere. Yield (pyiuCy) = 59% (1.928 g).
Yield (pyiuPr) = 51% (1.243 g).
The same products can be obtained in nearby quantitative yield
by reacting their conjugated acids [HpyiuCy]SO3CH3 and [HpyiuPr]-
SO3CH3 (see below) with a stoichiometric amount of potassium
tert-butoxide in methanol at room temperature. In a typical prep-
aration, to a solution containing 1.00 mmol of [HpyiuCy]SO3CH3
(0.412 g) or [HpyiuPr]SO3CH3 (0.331 g) in 30 mL of methanol
1.00 mmol (0.112 g) of solid K[OtBu] were added. The solvent
was then removed by evaporation at reduced pressure and diethyl-
ether (30 mL) was then added. The desired product was finally ob-
tained after filtration of the resulting solution and evaporation of
the solvent.
nected to
a programmable Thermo Scientific C25P cryostat
having a Phoenix II controlling unit. IR spectra of solid samples dis-
persed in KBr were recorded from 4000 to 450 cmꢀ1 on a Perkin-
Elmer Spectrum One spectrophotometer. IR spectra in solution
were recorded in the range 2200–1500 cmꢀ1 on 10ꢀ2 M solutions
of the complexes in dichloromethane, using cells with KBr win-
dows. 1H NMR, homodecoupled 1H NMR, 13C {1H} NMR, COSY,
NOESY, HSQC and HMBC spectra were recorded on a Bruker AC
200 and Bruker Avance 300 spectrometers. The NMR spectra of
the complexes were collected at variable temperature using CDCl3,
CD2Cl2, CD3NO2 and (CD3)2CO as solvents. 1H and 13C chemical
shifts (ppm) were referenced internally to undeuterated solvent
resonances and are quoted relative to tetramethylsilane
(d = 0 ppm). The numbering of pyridine hydrogen and carbon
atoms used for NMR assignments is reported in Schemes 1 and 2
for clarity. Conductivity measurements were carried out at 298 K
2.3. Characterization of pyiuCy
Elemental Anal. Calc. for C19H29N3O: C, 72.3; H, 9.27; N, 13.3.
Found: C, 72.2; H, 9.25; N, 13.2%. IR (KBr): 1669 cmꢀ1 mC@N
.
1H
3
NMR (CDCl3, 240 K): 8.52 (d, 1H, JHH = 4.9 Hz, pyridine-H6); 7.68
3
3
(t, 1H, JHH = 7.8 Hz, pyridine-H4); 7.36 (d, 1H, JHH = 7.8 Hz, pyri-
3
3
dine-H3); 7.18 (dd, 1H, JHH = 4.9 Hz, JHH = 7.8 Hz, pyridine-H5);
5.19 (s, 2H, CH2-O); 3.60 (d, 1H, JHH = 7.0 Hz, NH); 3.44 (s, br,
3
CH-Namine); 2.74 (s, br, CH-Nimine); 2.20–0.90 (m, 20H, cyclohexyl
groups). 13C {1H} NMR (CDCl3, 298 K): 158.4 pyridine-C2; 150.4 isou-
rea-C; 148.9 pyridine-C6; 136.2 pyridine-C4; 122.0 pyridine-C5;
on 10ꢀ3
M dimethylformamide solutions using a Radiometer
Copenhagen CDM 83 instrument and the molar conductivity values
were compared with literature data [11].
121.4 pyridine-C3; 67.2 CH2-O; 54.5 CH-Nimine; 48.9 CH-Namine
;
35.0–24.0 cyclohexyl groups.
2.2. Synthesis of 1,3-dicyclohexyl-2-((pyridin-2-yl)methyl)isourea
(pyiuCy) (C19H29N3O, MW = 315.45 g molꢀ1) and 1,3-diisopropyl-2-
((pyridin-2-yl)methyl)isourea (pyiuPr) (C13H21N3O, MW = 235.33 g
2.4. Characterization of pyiuPr
molꢀ1
)
Elemental Anal. Calc. for C13H21N3O: C, 66.4; H, 8.99; N, 17.9.
Found: C, 66.2; H, 8.97; N, 17.8%. IR (KBr): 1658 cmꢀ1 mC@N
.
1H
3
To a solution containing 2-pyridinemethanol (1.131 g, 10.36
mmol) and an equimolar amount of the proper carbodiimide
(2.138 g of dicyclohexylcarbodiimide for pyiuCy; 1.6 mL of diiso-
propylcarbodiimide for pyiuPr) in 100 mL of dichloromethane a
catalytic amount of copper(I) iodide (0.141 g, 0.74 mmol) was
added and the resulting reaction mixture was kept under vigorous
NMR (CDCl3, 240 K): 8.54 (d, 1H, JHH = 5.1 Hz, pyridine-H6); 7.67
3
3
(t, 1H, JHH = 7.9 Hz, pyridine-H4); 7.40 (d, 1H, JHH = 7.9 Hz, pyri-
3
3
dine-H3); 7.19 (dd, 1H, JHH = 5.1 Hz, JHH = 7.9 Hz, pyridine-H5);
5.19 (s, 2H, CH2-O); 3.83 (m, slightly br, 1H, (CH3)2CH-Namine);
3
3.56 (s, br, 1H, NH); 3.18 (sept, slightly br, JHH = 6.5 Hz,
(CH3)2CH-Nimine); 1.12 (d, 6H, JHH = 6.0 Hz, (CH3)2CH-Namine); 1.07
3
Scheme 1. Synthesis of pyiuCy and pyiuPr and of their conjugated acids [HpyiuCy]SO3CH3 and [HpyiuPr]SO3CH3.

68
M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
Scheme 2. Synthesis of the palladium(II) complexes 1Cy, 1Pr, 2Cy, 2Pr, 3Cy and 3Pr
.
3
(d, 6H, 3JHH = 6.5 Hz, (CH3)2CH-Nimine). 13C {1H} NMR (CDCl3, 298 K):
158.0 pyridine-C2; 150.9 isourea-C; 148.9 pyridine-C6; 136.3 pyri-
dine-C4; 122.1 pyridine-C5; 121.6 pyridine-C3; 67.5 CH2-O; 46.1
CH-Nimine; 43.5 CH-Namine; 24.0, 23.4 (CH3)2CH.
H4); 7.66 (d, slightly br, 1H, JHH = 7.7 Hz, pyridine-H3); 7.44 (dd,
3
3
slightly br, 1H, JHH = 5.0 Hz, JHH = 7.7 Hz, pyridine-H5); 5.58 (s,
2H, CH2-O); 4.08 (s, br, CH-N); 2.72 (s, 3H, CH3SO3); 1.26 (d, 12H,
3JHH = 6.6 Hz, JHH = 7.7 Hz, (CH3)2CH-N). 1H NMR (CDCl3, 258 K):
3
9.33 (s, br, 1H, NH); 8.80–8.45 (m, br, 2H, NH and pyridine-H6);
7.93 (t, 1H, JHH = 7.7 Hz, pyridine-H4); 7.66 (d, slightly br, 1H,
3
2.5. Synthesis of [HpyiuCy]SO3CH3 (C20H33N3O4S, MW = 411.56 g
3JHH = 7.7 Hz, pyridine-H3); 7.48 (m, slightly br, 1H, pyridine-H5);
5.52 (s, 2H, CH2-O); 4.25–3.84 (m, br, 2H, CH-N); 2.72 (s, 3H,
CH3SO3); 1.23 (m, br, 12H, (CH3)2CH-N). 13C {1H} NMR (CDCl3,
298 K): 157.3 isourea-C; 152.1 pyridine-C2; 148.7 pyridine-C6;
138.1 pyridine-C4; 124.4 pyridine-C5; 124.1 pyridine-C3; 73.4 CH2-
O; 45.2 CH-N; 39.4 CH3SO3; 32.2–21.9 (CH3)2CH.
molꢀ1) and [HpyiuPr]SO3CH3 (C14H25N3O4S, MW = 331.43 g molꢀ1
)
To a solution containing 4.75 mmol of pyiuCy (1.498 g) or pyiuPr
(1.118 g) in dichloromethane (30 mL) methanesulfonic acid
(290 lL, 4.50 mmol) was added with a microsyringe at room tem-
perature. The solution was stirred at room temperature for 30 min
and subsequently the solvent was removed by evaporation under
reduced pressure. Diethylether (30 mL) was then added and the
resulting mixture was left under vigorous stirring overnight. In
the case of the cyclohexyl-substituted product a white solid sepa-
rated out, which was filtered, washed with about 10 mL of diethyl-
ether and dried under vacuum. Yield = 1.779 g, 91%. The isopropyl-
derivative was instead a yellowish oil, which was washed three
times with about 20 mL of diethylether and dried under vacuum.
Yield = 1.401 g, 89%. Both the products resulted stable for many
months if stored at ꢀ20 °C. The slow cooling of dichloromethane/
diethyl ether solutions of [HpyiuCy]SO3CH3 afforded crystals suit-
able for X-ray analysis.
2.8. Synthesis of PdCl2(pyiuCy) (1Cy) (C19H29Cl2N3OPd, MW = 492.78 g
molꢀ1) and PdCl2(pyiuPr) (1Pr) (C13H21Cl2N3OPd, MW = 412.65 g
molꢀ1
)
To a solution of cis-PdCl2(CH3CN)2 (1.00 mmol, 0.259 g) in
20 mL of CH2Cl2 a solution containing 1.00 mmol of freshly pre-
pared pyiuCy or pyiuPr in 10 mL of CH2Cl2 was slowly added. The
resulting reaction mixture was left under stirring at room temper-
ature for 3 h, then the solution was concentrated to about 5 mL by
evaporation under reduced pressure and diethylether (10 mL) was
slowly added. The yellow solid obtained was collected by filtration.
The product was purified by dissolving it in dimethylformamide
(3 mL). The DMF solution was then filtered and diethylether was
added until a yellow solid separated out, which was collected by
filtration, washed with diethylether and dried under vacuum. Yield
(1Cy) = 86%, 0.424 g. Yield (1Pr) = 80%, 0.330 g. The slow cooling of
acetone/nitromethane/diethyl ether solutions of 1Cy afforded crys-
tals suitable for X-ray analysis.
2.6. Characterization of [HpyiuCy]SO3CH3
Elemental Anal. Calc. for C20H33N3O4S: C, 58.4; H, 8.08; N, 10.2.
Found: C, 58.2; H, 8.05; N, 10.2%. IR (KBr): 3422 cmꢀ1, 3327 cmꢀ1
m
N–H; 1644 cmꢀ1 mC@N KM (dimethylformamide, 298 K) = 51 ohmꢀ1
.
molꢀ1 cm2. 1H NMR (CDCl3, 298 K): 9.24 (s, br, 2H, NH); 8.57 (d, 1H,
3JHH = 5.1 Hz, pyridine-H6); 7.82 (t, 1H, JHH = 7.8 Hz, pyridine-H4);
3
3
3
2.9. Characterization of 1Cy
7.54 (d, 1H, JHH = 7.8 Hz, pyridine-H3); 7.38 (dd, 1H, JHH = 5.1 Hz,
3JHH = 7.8 Hz, pyridine-H5); 5.44 (s, 2H, CH2-O); 3.72 (s, br, 2H, CH-
N); 2.72 (s, 3H, CH3SO3); 1.95–1.00 (m, 20H, cyclohexyl groups). 1H
NMR (CDCl3, 253 K): 9.80 (s, br, 1H, NH); 8.87 (d, slightly br, 1H,
Elemental Anal. Calc. for C19H29Cl2N3OPd: C, 46.3; H, 5.93; Cl,
14.4; N, 8.53. Found: C, 46.2; H, 5.90; Cl, 14.4; N, 8.50%. IR (KBr):
3JHH = 7.3 Hz, NH); 8.55 (d, 1H, JHH = 5.1 Hz, pyridine-H6); 7.85 (t,
3
1612 cmꢀ1 mC@N
.
1H NMR (acetone-d6, 241 K): 8.79 (d, 1H,
3
3
3
3JHH = 5.6 Hz, pyridine-H6); 8.19 (t, 1H, JHH = 7.8 Hz, pyridine-H4);
1H, JHH = 7.8 Hz, pyridine-H4); 7.56 (d, 1H, JHH = 7.8 Hz, pyridine-
3
3
3
2
H3); 7.41 (dd, 1H, JHH = 5.1 Hz, JHH = 7.8 Hz, pyridine-H5); 5.41 (s,
2H, CH2-O); 4.13–3.26 (m, br, 2H, CH-N); 2.73 (s, 3H, CH3SO3);
2.10–0.95 (m, 20H, cyclohexyl groups). 13C {1H} NMR (CDCl3,
298 K): 152.5 pyridine-C2; 157.8 isourea-C; 148.9 pyridine-C6;
138.1 pyridine-C4; 124.5 pyridine-C5; 124.4 pyridine-C3; 74.0
CH2-O; 52.1 CH-N (br); 39.7 CH3SO3; 32.2–23.8 cyclohexyl rings.
7.96 (d, 1H, JHH = 7.8 Hz, pyridine-H3); 7.91 (d, 1H, JHH = 12.7 Hz,
3
3
CH2-O); 7.74 (dd, 1H, JHH = 5.6 Hz, JHH = 7.8 Hz, pyridine-H5);
3
2
5.98 (d, 1H, JHH = 7.5 Hz, NH); 5.82 (d, 1H, JHH = 12.7 Hz, CH2-O);
3.61 (m, br, 1H, CH-Namine); 3.11 (m, br, 1H, CH-Nimine); 2.00–0.80
(m, 20H, cyclohexyl groups). 1H NMR (acetone-d6, 311 K): 8.81 (d,
3
3
1H, JHH = 5.6 Hz, pyridine-H6); 8.11 (t, 1H, JHH = 7.8 Hz, pyridine-
3
H4); 7.97 (s, very br, CH2-O); 7.85 (d, 1H, JHH = 7.8 Hz, pyridine-
H3); 7.68 (dd, 1H, JHH = 5.6 Hz, JHH = 7.8 Hz, pyridine-H5); 5.74
(s, very br, 1H, CH2-O); 5.50 (d, slightly br, 1H, JHH = 7.5 Hz, NH);
3
3
2.7. Characterization of [HpyiuPr]SO3CH3
3
Elemental Anal. Calc. for C14H25N3O4S: C, 50.7; H, 7.60; N, 12.7.
3.63 (m, slightly br, 1H, CH-Namine); 2.85 (m, slightly br, 1H,
Found: C, 50.5; H, 7.60; N, 12.6%. IR (KBr): 3422 cmꢀ1, 3327 cmꢀ1
CH-Nimine); 2.00–0.80 (m, 20H, cyclohexyl groups). 1H NMR (CD3NO2,
3
4
mN–H
;
1647 cmꢀ1 mC@N
.
KM (dimethylformamide, 298 K) = 52
298 K): 8.75 (d, 1H, JHH = 5.6 Hz, JHH = 1.5 Hz, pyridine-H6); 8.05
ohmꢀ1 molꢀ1 cm2. 1H NMR (CDCl3, 328 K): 8.89 (s, br, 2H, NH);
(td, 1H, JHH = 7.7 Hz, JHH = 1.5 Hz, pyridine-H4); 7.89 (d, 1H,
3
4
3
3
8.66 (s, br, 1H, pyridine-H6); 7.89 (t, 1H, JHH = 7.7 Hz, pyridine-
2JHH = 13.0 Hz, CH2-O); 7.73 (d, 1H, JHH = 7.7 Hz, pyridine-H3);

M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
69
3
3
3
3
7.61 (dd, 1H, JHH = 5.6 Hz, JHH = 7.7 Hz, pyridine-H5); 5.67 (d, 1H,
7.39 (dd, 1H, JHH = 5.4 Hz, JHH = 7.8 Hz, pyridine-H5); 5.49 (s, br,
2JHH = 13.0 Hz, CH2-O); 4.78 (d, 1H, JHH = 7.6 Hz, NH); 3.64 (m,
1H, CH2-O); 3.93 (d, 1H, JHH = 7.1 Hz, NH); 3.42 (s, br, 1H,
3
3
1H, CH-Namine); 2.86 (m, 1H, CH-Nimine); 2.50–1.00 (m, 20H, cyclo-
hexyl groups). 13C {1H} NMR (CD3NO2, 248 K): 155.3, 154.9 not H-
bonded carbon atoms; 154.0, 141.6, 128.3, 128.1 pyridine-CH; 72.0
CH-O; 54.0–25.0 cyclohexyl rings.
CH-Namine); 3.00 (s, br, 1H, CH-Nimine); 1.91 (s, 3H, CH3COO); 1.83
(s, 3H, CH3COO); 2.50–0.80 (m, 20H, cyclohexyl groups).
2.13. Characterization of 2Pr
2.10. Characterization of 1Pr
Elemental Anal. Calc. for C17H27N3O5Pd: C, 44.4; H, 5.92; N, 9.14.
Found: C, 44.5; H, 5.95; N, 9.15%. IR (KBr): 1625 cmꢀ1 (broad sig-
Elemental Anal. Calc. for C13H21Cl2N3OPd: C, 37.8; H, 5.13; Cl,
nal) mC@N and
mC@N and mC@O
m
.
C@O. IR (CH2Cl2): 1630 cmꢀ1 (shoulder), 1616 cmꢀ1
2
17.2; N, 10.2. Found: C, 37.8; H, 5.10; Cl, 17.1; N, 10.1%. IR (KBr):
1H NMR (CDCl3, 239 K): 9.58 (d, 1H, JHH = 13.0 Hz,
1613 cmꢀ1 mC@N
.
1H NMR (acetone-d6, 270 K): 8.80 (d, 1H,
CH2-O); 8.67 (d, 1H, JHH = 5.5 Hz, pyridine-H6); 7.87 (t, 1H,
3
3JHH = 5.5 Hz, pyridine-H6); 8.18 (t, 1H, JHH = 7.7 Hz, pyridine-H4);
3JHH = 7.6 Hz, pyridine-H4); 7.51 (d, 1H, JHH = 7.6 Hz, pyridine-H3);
3
3
2
3
3
3
8.07 (d, 1H, JHH = 12.7 Hz, CH2-O); 7.96 (d, 1H, JHH = 7.7 Hz, pyri-
7.41 (dd, 1H, JHH = 5.5 Hz, JHH = 7.6 Hz, pyridine-H5); 5.50 (d, 1H,
3
3
3
dine-H3); 7.74 (dd, 1H, JHH = 5.5 Hz, JHH = 7.7 Hz, pyridine-H5);
2JHH = 13.0 Hz, CH2-O); 4.12 (d, 1H, JHH = 6.6 Hz, NH); 3.77 (m,
2
3
5.83 (d, 1H, JHH = 12.7 Hz, CH2-O); 5.81 (d, 1H, JHH = 6.5 Hz, NH);
1H, CH-Namine); 2.87 (m, 1H, CH-Nimine); 1.89 (s, 3H, CH3COO);
3
3.99 (m, 1H, CH-Namine); 3.22 (sept, 1H, JHH = 6.4 Hz, CH-Nimine);
1.81 (s, 3H, CH3COO); 2.30–0.80 (m, 12H, (CH3)2CH). 1H NMR
3
3
3
1.81 (d, 3H, JHH = 6.4 Hz, (CH3)2CH-Nimine); 1.30 (d, 3H, JHH
=
(CDCl3, 317 K): 8.76 (d, 1H, JHH = 5.5 Hz, pyridine-H6); 7.83 (t, 1H,
6.4 Hz, (CH3)2CH-Nimine); 1.19 (d, 3H, 3JHH = 6.4 Hz, (CH3)2CH-Namine);
3JHH = 7.7 Hz, pyridine-H4); 7.48 (d, 1H, JHH = 7.7 Hz, pyridine-H3);
3
3
3
3
1.05 (d, 3H, JHH = 6.4 Hz, (CH3)2CH-Namine). 1H NMR (acetone-d6,
7.37 (dd, 1H, JHH = 5.5 Hz, JHH = 7.6 Hz, pyridine-H5); 4.00 (d, 1H,
3
3
318 K): 8.81 (d, 1H, JHH = 5.5 Hz, pyridine-H6); 8.13 (t, 1H,
3JHH = 6.9 Hz, NH); 3.81 (m, 1H, CH-Namine); 2.96 (sept, 1H, JHH
=
3JHH = 7.7 Hz, pyridine-H4); 7.88 (d, 1H, JHH = 7.7 Hz, pyridine-H3);
6.6 Hz, CH-Nimine); 1.89 (s, 3H, CH3COO); 1.81 (s, 3H, CH3COO);
3
3
3
3
3
7.69 (dd, 1H, JHH = 5.5 Hz, JHH = 7.7 Hz, pyridine-H5); 5.51 (d, 1H,
1.62 (d, br, 6H, JHH = 6.6 Hz, (CH3)2CH-Nimine); 1.05 (d, 6H, JHH
6.3 Hz, (CH3)2CH-Namine).
=
3JHH = 6.5 Hz, NH); 4.01 (m, 1H, CH-Namine); 3.28 (sept, 1H,
3JHH = 6.4 Hz, CH-Nimine); 1.57 (s, br, 6H, JHH = 6.4 Hz, (CH3)2CH-Ni-
3
mine); 1.16 (d, 6H, 3JHH = 6.4 Hz, (CH3)2CH-Namine). 1H NMR (CD3NO2,
2.14. Synthesis of Pd(CH3)Cl(pyiuCy) (3Cy) (C20H32ClN3OPd,
3
298 K): 8.74 (d, 1H, JHH = 5.7 Hz, pyridine-H6); 8.06 (t, 1H,
MW = 472.36 g molꢀ1) and Pd(CH3)Cl(pyiuPr) (3Pr) (C14H24ClN3OPd,
3JHH = 7.7 Hz, pyridine-H4); 8.01 (d, 1H, JHH = 12.9 Hz, CH2-O);
MW = 392.23 g molꢀ1
)
2
3
3
7.75 (d, 1H, JHH = 7.7 Hz, pyridine-H3); 7.63 (dd, 1H, JHH = 5.5 Hz,
3JHH = 7.7 Hz, pyridine-H5); 5.69 (d, 1H, JHH = 12.9 Hz, CH2-O);
To a solution of Pd(CH3)Cl(COD) (1.00 mmol, 0.265 g) in 20 mL
of CH2Cl2 a solution containing 1.00 mmol of freshly prepared
pyiuCy or pyiuPr in 10 mL of CH2Cl2 was slowly added. The result-
ing reaction mixture was left under stirring at room temperature
for 4 h, then the solvent was removed by evaporation under re-
duced pressure and diethylether (10 mL) was added. The solid
which separated out was filtered, washed with diethylether
(10 mL) and dried under vacuum. The products were purified by
slowly cooling saturated dichloromethane/diethylether solutions
from room temperature to ꢀ25 °C. Yield (3Cy) = 75%, 0.354 g. Yield
(3Pr) = 76%, 0.298 g.
2
3
4.78 (d, 1H, JHH = 7.5 Hz, NH); 3.97 (m, 1H, CH-Namine); 3.14 (sept,
3
3
1H, JHH = 6.4 Hz, CH-Nimine); 1.81 (d, 3H, JHH = 6.4 Hz, (CH3)2
3
CH-Nimine); 1.30 (d, 3H, JHH = 6.4 Hz, (CH3)2CH-Nimine); 1.19 (d,
3
3
3H, JHH = 6.4 Hz, (CH3)2CH-Namine); 1.05 (d, 3H, JHH = 6.4 Hz,
(CH3)2 CH-Namine). 13C {1H} NMR (CD3NO2, 298 K): 155.8, 155.5
not H-bonded carbon atoms; 154.2, 141.7, 128.3, 128.1 pyridine-
CH; 72.3 CH2-O; 53.4, 46.8 CH-N; 26.2, 23.7, 23.3, 22.6 (CH3)2CH-N.
2.11. Synthesis of Pd(OCOCH3)2(pyiuCy) (2Cy) (C23H35N3O5Pd,
MW = 539.96 g molꢀ1) and Pd(OCOCH3)2(pyiuPr) (2Pr
)
(C17H27N3O5Pd, MW = 459.83 g molꢀ1
)
2.15. Characterization of 3Cy
To a solution of palladium(II) acetate (1.00 mmol, 0.225 g) in
20 mL of CH2Cl2 a solution containing 1.00 mmol of freshly pre-
pared pyiuCy or pyiuPr in 10 mL of CH2Cl2 was slowly added. The
resulting reaction mixture was left under stirring at room temper-
ature for 4 h, then the solvent was removed by evaporation under
reduced pressure and diethylether (10 mL) was added. The solid
which separated out was filtered, washed with diethylether
(10 mL) and dried under vacuum. Yield (2Cy) = 90%, 0.486 g. Yield
(2Pr) = 88%, 0.407 g.
Elemental Anal. Calc. for C20H32ClN3OPd: C, 50.9; H, 6.83; Cl,
7.51; N, 8.90. Found: C, 50.8; H, 6.80; Cl, 7.50; N, 8.90%. IR (KBr):
1630 cmꢀ1 mC@N
.
1H NMR (CDCl3, 225 K): 8.54 (d, 1H, 3JHH = 5.5 Hz,
2
pyridine-H6); 8.02 (d, 1H, JHH = 12.5 Hz, CH2-O); 7.93 (t, 1H,
3JHH = 7.7 Hz, pyridine-H4); 7.62 (d, 1H, JHH = 7.7 Hz, pyridine-H3);
3
3
3
7.49 (dd, 1H, JHH = 5.5 Hz, JHH = 7.7 Hz, pyridine-H5); 5.32 (d, 1H,
2JHH = 12.5 Hz, CH2-O); 3.60 (d, 1H, JHH = 6.4 Hz, NH); 3.24 (m,
3
1H, CH-Namine); 2.57 (m, 1H, CH-Nimine); 2.30–0.90 (m, 20H, cyclo-
hexyl groups); 0.78 (s, 3H, Pd-CH3).
2.12. Characterization of 2Cy
2.16. Characterization of 3Pr
Elemental Anal. Calc. for C23H35N3O5Pd: C, 51.2; H, 6.53; N, 7.78.
Found: C, 51.3; H, 6.55; N, 7.80%. IR (KBr): 1610 cmꢀ1 (broad sig-
Elemental Anal. Calc. for C14H24ClN3OPd: C, 42.9; H, 6.17; Cl,
9.04; N, 10.7. Found: C, 43.0; H, 6.20; Cl, 9.05; N, 10.7%. IR (KBr):
nal) mC@N and
mC@N and mC@O
m
.
C@O. IR (CH2Cl2): 1630 cmꢀ1 (shoulder), 1609 cmꢀ1
2
1H NMR (CDCl3, 242 K): 9.64 (d, 1H, JHH = 13.4 Hz,
1629 cmꢀ1
m
C@N. 1H NMR (CD2Cl2, 241 K): 8.43 (d, 1H, 3JHH = 5.5 Hz,
3
3
CH2-O); 8.68 (d, 1H, JHH = 5.4 Hz, pyridine-H6); 7.87 (t, 1H,
pyridine-H6); 7.83 (t, 1H, JHH = 7.7 Hz, pyridine-H4); 7.81 (d, 1H,
3JHH = 7.8 Hz, pyridine-H4); 7.52 (d, 1H, JHH = 7.8 Hz, pyridine-H3);
2JHH = 12.0 Hz, CH2-O); 7.51 (d, 1H, JHH = 7.7 Hz, pyridine-H3);
3
3
3
3
3
3
7.40 (dd, 1H, JHH = 5.4 Hz, JHH = 7.8 Hz, pyridine-H5); 5.51 (d, 1H,
2JHH = 13.4 Hz, CH2-O); 4.03–3.15 (m, 3H, NH and CH-Namine and
CH-Nimine); 1.92 (s, 3H, CH3COO); 1.83 (s, 3H, CH3COO); 2.50–0.80
(m, 20H, cyclohexyl groups). 1H NMR (CDCl3, 271 K): 9.64 (s, br,
7.39 (dd, 1H, JHH = 5.5 Hz, JHH = 7.7 Hz, pyridine-H5); 5.25 (d, 1H,
2JHH = 12.0 Hz, CH2-O); 3.80–3.52 (m, 2H, NH and CH-Namine); 2.99
3
3
(sept, 1H, JHH = 6.4 Hz, CH-Nimine); 1.67 (d, 3H, JHH = 6.4 Hz,
3
(CH3)2CH-Nimine); 1.10 (d, 3H, JHH = 6.4 Hz, (CH3)2CH-Nimine); 1.02
3
3
3
1H, CH2-O); 8.71 (d, 1H, JHH = 5.4 Hz, pyridine-H6); 7.85 (t, 1H,
(d, 3H, JHH = 6.4 Hz, (CH3)2CH-Namine); 0.89 (d, 3H, JHH = 6.4 Hz,
(CH3)2CH-Namine); 0.57 (s, 3H, Pd-CH3).
3
3JHH = 7.8 Hz, pyridine-H4); 7.51 (d, 1H, JHH = 7.8 Hz, pyridine-H3);

70
M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
2.17. Crystal structure determinations
3. Results and discussion
The crystal data of compounds [HpyiuCy]SO3CH3 and 1Cy were
3.1. Synthesis and characterization of the ligands
collected at room temperature using a Nonius Kappa CCD diffrac-
tometer with graphite monochromated Mo Ka radiation. The data
The ligands pyiuCy and pyiuPr were obtained with yields higher
than 50% by reacting 2-pyridinemethanol with an equimolar
amount of dicyclohexylcarbodiimide or diisopropylcarbodiimide
in dichloromethane at room temperature in the presence of a cat-
alytic amount of copper(I) iodide, as sketched in Scheme 1. The
yields of the reactions were not meaningfully improved on chang-
ing the reaction conditions. The reaction between a benzyl alcohol
and a carbodiimide to obtain the corresponding isourea is reported
in the literature [25]. The commonly used catalysts are copper(I) or
copper(II) salts, but in the case of 2-pyridinemethanol as reactant
preliminary experiments showed the complete inefficacy of Cu(II)
salts as catalysts for such a reaction.
sets were integrated with the Denzo-SMN package [12] and cor-
rected for Lorentz, polarization and absorption effects (SORTAV
[13]). The structures were solved by direct methods (SIR97 [14])
and refined using full-matrix least-squares with all non-hydrogen
atoms anisotropically. For structure [HpyiuCy]SO3CH3 the hydro-
gens were refined isotropically except those of the SO3CH3 anion
which were included in the calculated positions. For structure 1Cy
the hydrogens were included on calculated positions, riding on
their carrier atoms, except the aminic hydrogen which was refined
isotropically. All calculations were performed using SHELXL-97 [15]
and PARST [16] implemented in WINGX [17] system of programs.
The crystal data are given in Table 1.
The elemental analyses of pyiuCy and pyiuPr are in agreement
with the proposed formulations. The IR spectra show a quite strong
band attributable to the C@N stretching, at 1669 cmꢀ1 for pyiuCy
2.18. Theoretical calculations
and at 1658 cmꢀ1 for pyiuPr
.
The 1H NMR spectra show in the high-frequency region four sig-
nals attributable to the pyridine ring. The O-bonded CH2 group cor-
responds in both the ligands to a sharp singlet centered at
5.19 ppm. In the spectral region comprised between 4.0 and
2.5 ppm the signals corresponding to the NH and CH-N protons
are observable. In both the compounds the CH bonded to Namine
resonates at higher frequency with respect to that bonded to
The computational geometry optimization of the complexes
was carried out using the hybrid DFT B3PW91 method [18] with-
out symmetry constrains in combination with a polarized triple-f
quality basis set composed by the 6-311G(d,p) basis set on the
light atoms and the LANL2TZ(f) basis set on the metal center
[19]. Implicit solvation was added by using the C-PCM model for
acetone (e = 20.7) [20]. The ‘‘restricted’’ formalism was applied in
all calculations. The geometries were initially refined in vacuo by
using the M06 DFT method [21] in combination with the LACVP⁄⁄
[22] basis set and the stationary points were characterized as true
minima by IR simulation. The software used for M06 calculations
was the SPARTAN 08 [23], while GAUSSIAN 09 [24] was used for
B3PW91 calculations. The simulations were performed on a local
Intel-based x86-64 workstation and at CINECA (Centro Italiano di
Supercalcolo, Bologna, Italy) using a IBM P6-575 workstation
equipped with 64-bit IBM Power6 processors.
N
imine. The 1H NMR spectra were recorded at 240 K in order to
distinguish these groups. In the low-frequency region the other
signals of the cyclohexyl or isopropyl groups are observable.
The 13C {1H} NMR spectra show in the high-frequency region six
signals, five attributable to the pyridine ring and one to the isourea
group carbon atom, which falls around 150 ppm. The assignment
was based on HSQC and HMBC spectra. The O-bonded CH2 corre-
sponds to a singlet around 67 ppm. In the range comprised be-
tween 50.0 and 40.0 ppm the CH-N groups are observable and,
differently from the 1H NMR resonances, the CH bonded to Nimine
is more down-shifted with respect to the CH-Namine group. Finally,
the other signals corresponding to the cyclohexyl or isopropyl
groups are present in the low-frequency region.
Table 1
Crystallographic data for [HpyiuCy]SO3CH3 and 1Cy
.
The compounds pyiuCy and pyiuPr are scarcely stable. Addition
of a stoichiometric amount of methanesulfonic acid to these neu-
tral compounds in dichloromethane caused the formation of the
conjugated acids [HpyiuCy]SO3CH3 and [HpyiuPr]SO3CH3 (see
Scheme 1), which are on the contrary stable for many months if
stored at ꢀ25 °C.
Compound
[HpyiuCy]SO3CH3
1Cy
Formula
M
Space group
Crystal system
a (Å)
C20H33N3O4S
411.55
P21/n
monoclinic
15.1881(3)
9.9542(2)
16.1021(3)
112.863(1)
2243.14(8)
4
C19H29Cl2N3Opd
492.75
P21/c
monoclinic
16.6464(4)
7.2914(2)
18.7464(6)
111.306(1)
2119.8(1)
4
b (Å)
c (Å)
[HpyiuCy]SO3CH3 and [HpyiuPr]SO3CH3 behave as 1:1 electro-
lytes in DMF solution and their elemental analyses are in agree-
ment with the proposed formulations. The IR spectra show bands
attributable to the N–H stretching in the region comprised be-
tween 3450 and 3300 cmꢀ1. The C@N stretching corresponds to a
band at 1644 cmꢀ1 for [HpyiuCy]SO3CH3 and at 1647 cmꢀ1 for
[HpyiuPr]SO3CH3.
b (°)
U (Å3)
Z
T (K)
295
1.219
888
1.73
295
1.544
1008
11.40
Dcalc (g cmꢀ3
F(000)
)
l(Mo K
a
) (cmꢀ1
)
The 1H NMR spectra of these species in CDCl3 at 298 K show in
the high-frequency region, besides the signals of the pyridine ring,
a broad singlet attributable to the NH protons. If the temperature is
lowered below 260 K, this singlet separates in two signals, the first
one above 9.3 ppm and quite broad, the second one below 8.8 ppm
and more sharp. This fluxional behavior of [HpyiuCy]SO3CH3 and
[HpyiuPr]SO3CH3 is attributable to proton exchange, which be-
comes slow on the NMR timescale at low temperature. The chem-
ical shifts of the N-bonded hydrogen atoms are influenced by
hydrogen bonding, as observed also by single-crystal X-ray diffrac-
tion (see below). In both the compounds the CH2-O 1H NMR signals
are singlets around 5.5–5.4 ppm. In the aliphatic region the signals
Measured reflections
Unique reflections
Rint
Observed reflections
[I P 2r(I)]
9650
5367
0.0214
3943
14449
5010
0.0414
3933
hmin–hmax (°)
hkl ranges
3.56–28.00
3.64–27.91
ꢀ20, 20; ꢀ12, 13; ꢀ21, ꢀ21, 21; ꢀ9, 9; ꢀ24,
21
24
R(F2) (observed reflections) 0.0493
0.0367
0.0860
239
wR(F2) (all reflections)
Number of variables
Goodness of fit
0.1457
374
1.020
1.054
D
qmax
;
D
qmin (e Åꢀ3
)
0.29; ꢀ0.37
0.51; ꢀ0.68
CCDC Deposition Number
CCDC 853518
CCDC 853519

M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
71
Table 2
of the N-bonded substituents and of the methanesulfonate coun-
Selected geometrical parameters for [HpyiuCy]SO3CH3 (Å and °).
ter-ion are observable.
The 13C {1H} NMR spectra of the two compounds show in the
high-frequency region, besides the pyridine C-atoms, the isourea
carbon around 158–157 ppm. The other signals present in the 13C
{1H} NMR spectra are those of the O-bonded methylene and of
the aliphatic substituents on the nitrogen atoms.
Distances
Angles
C1–O1
C2–O1
C1–N2
C1–N3
C8–N2
C14–N3
1.329(2)
1.457(2)
1.315(3)
1.314(2)
1.474(2)
1.471(3)
C1–O1–C2
O1–C1–N2
O1–C1–N3
N2–C1–N3
C1–N2–C8
C1–N3–C14
120.0(1)
121.7(1)
114.6(1)
123.7(2)
124.2(1)
125.8(2)
Crystals suitable for X-ray structure diffraction have been ob-
tained for [HpyiuCy]SO3CH3. ORTEP [26] view is shown in Fig. 1
and a selection of bond distances and angles is given in Table 2.
The protonation of the neutral pyridine–isourea ligands by
methanesulfonic acid occurs at the Nimine atom of the isourea dent.
The similarity of C1–N2 and C1–N3 distances of 1.315(3) and
1.314(2) Å, respectively, reveals a significant delocalization of the
positive charge as well as of the double bond on the N2–C1–N3
Table 3
Hydrogen bond parameters for [HpyiuCy]SO3CH3.
Hydrogen bonds Symm. op.
D–H
(Å)
D. . .A
(Å)
H. . .A
(Å)
D–H. . .A
(°)
moiety. The crystal packing is characterized by chains of alternate
[HpyiuCy +
]
cations and SO3CH3 anions linked by strong intermo-
N2–H2N. . .O2
N3–H3N. . .O3
0.88(2) 2.880(2) 2.03(2) 165(2)
0.86(2) 2.817(2) 1.98(2) 166(2)
ꢀ
1/2 ꢀ x, 1/2 + y,
lecular N2–H. . .O2 and N3–H. . .O3 hydrogen bonds (Table 3). Other
short C–H. . .O interactions between cations and anions are in-
volved in the reinforcement of the crystal packing (Table 3).
Reaction of [HpyiuCy]SO3CH3 and [HpyiuPr]SO3CH3 with potas-
sium tert-butoxide in methanol at room temperature allows
obtaining again with nearby quantitative yields the conjugated
bases pyiuCy and pyiuPr (see Scheme 1).
1/2 ꢀ z
C8–H8. . .O3
1/2 ꢀ x, 1/2 + y,
1/2 ꢀ z
0.99(2) 3.332(2) 2.62(2) 129(2)
C4–H4. . .O4
C2–H2A. . .O2
1 ꢀ x, ꢀ1 ꢀ y, 1 ꢀ z 0.94(3) 3.293(3) 2.42(3) 155(2)
1 ꢀ x, ꢀ1 ꢀ y, 1 ꢀ z 0.96(2) 3.443(2) 2.49(2) 173(2)
(2Cy), Pd(OAc)2(pyiuPr) (2Pr), Pd(CH3)Cl(pyiuCy) (3Cy) and Pd(CH3)
Cl(pyiuPr) (3Pr), as depicted in Scheme 2. The dichloro-complexes
1
while they are quite soluble in more polar solvents such as acetone
and nitromethane and very soluble in dimethylformamide. The
complexes 2Cy, 2Pr, 3Cy and 3Pr are instead very soluble in all these
solvents.
3.2. Coordination of the pyridine–isourea ligands to the Pd(II) center
Cy and 1Pr are poorly soluble in dichloromethane and chloroform,
The ligands pyiuCy and pyiuPr easily react with the precursors
cis-PdCl2(NCCH3)2, Pd(II) acetate and Pd(CH3)Cl(COD) in dichloro-
methane at room temperature to form the corresponding com-
plexes PdCl2(pyiuCy) (1Cy), PdCl2(pyiuPr) (1Pr), Pd(OAc)2(pyiuCy
)
Fig. 1. ORTEP view of compound [HpyiuCy]SO3CH3 showing the thermal ellipsoids at 30% probability level.

72
M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
Fig. 2. Section of 1H NOESY spectrum of 1Pr in CD3NO2 at 298 K.
Fig. 3. 1H NMR spectra in acetone-d6 of 1Pr at 270 and 318 K.
All these complexes are not conductive in dimethylformamide
solution and their elemental analyses agree with the proposed for-
mulations. The IR spectra show bands attributable to the C@N
stretching in the range 1630–1612 cmꢀ1. For all the complexes this

M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
73
stretching falls at wavenumbers lower than those recorded for the
corresponding free ligands. In the case of the diacetato-derivatives
2
C@O bonds and the resulting wide band cannot be completely re-
solved also by recording the IR spectra of the samples in CH2Cl2
solution.
The signals corresponding to the isopropyl fragments are
strongly affected by the temperature. If the temperature is suffi-
ciently low both the isopropyl fragments show two distinct methyl
resonances, while on rising the temperature the two methyl of the
same group give as single signal. This result can be attributed to a
slow inversion at the N atom of the amine unit at low temperature,
or to a slow inversion of the seven-membered ring. As an example,
Fig. 3 reports the 1H NMR spectra of 1Pr in acetone-d6 at 270 and
318 K: the two CH3 signals of the Namine-bonded isopropyl group
(1.19 and 1.05 ppm at 270 K) become a single sharp doublet at
Cy and 2Pr the C@N stretching is superposed with those due to the
The 1H NMR spectra of all the complexes show in the high-fre-
quency region the signals due to the pyridine ring. The isourea NH
resonance strongly depends upon the solvent: in acetone and
nitromethane the chemical shift value is comprised between 6.0
and 4.7 ppm, while the same signal falls between 4.1 and
3.5 ppm in chloroform or dichloromethane. On the basis of the sca-
lar coupling with this NH group the 1H NMR signals of the aliphatic
groups bonded to Nimine and Namine of the isourea dent have been
attributed. The CH2-O protons correspond in all the complexes to
1.16 ppm at 318 K, while the two CH3 doublets of the Nimine
-
bonded isopropyl group (1.81 and 1.30 ppm at 270 K) coalesce in
broad, unresolved peak centered at 1.57 ppm at 318 K. Another
example of the same behavior is observable in the NMR spectra
of 2Pr in CDCl3 reported in Fig. 4. At 239 K four separate CH3 signals
are observable in the range comprised between 2.30 and 1.80 ppm,
while a broad signal at 1.62 ppm and a sharp doublet at 1.05 ppm
are present in the spectrum recorded at 317 K, corresponding to
the Nimine- and Namine-bonded isopropyl groups, respectively.
Other NMR signals observable in the aliphatic region of the 1H
NMR spectra are the two singlets corresponding to the two aceta-
to-ligands in 2Cy and 2Py and the low-frequency singlet due to the
Pd-CH3 group in 3Cy and 3Pr. The NOESY cross peak between this
signal and that of pyridine-H6 allows concluding that the Pd-
bonded methyl group and the pyridine ring are mutually in cis po-
sition, as sketched in Scheme 2.
2
a AB spin system if the temperature is sufficiently low, with JHH
coupling constant values comprised between 12.0 and 13.5 Hz.
The coordinated ligands have however a fluxional behavior and
the CH2-O signals become broader or not detectable on rising the
temperature.
These NMR signals agree with a coordination mode where the
ligands act as bidentate and coordinate the metal fragments
through the pyridine and the isourea groups. The coordinating
nitrogen atom of the isourea dent has been identified on the basis
of low-temperature NOESY experiments. The cross-peak between
one of the N-bonded isopropyl fragments and the pyridine-H6 in
1
Pr, 2Pr and 3Pr (see Fig. 2 as example) allows concluding that the
The coordination mode of the pyridine–isourea ligands towards
palladium(II) proposed on the basis of NMR measurements has
been confirmed by the X-ray structure determination of 1Cy and
an ORTEP [26] view of the molecular structure of this complex is
isourea group coordinates the metal center through the imine
nitrogen, as already observed for other Pd(II)–isourea complexes
(see Scheme 2) [4].
Fig. 4. 1H NMR spectra in CDCl3 of 2Pr at 239 and 317 K.

74
M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
Table 4
shown in Fig. 5. Selected bond distances and angles are listed in Ta-
ble 4. The geometry around Pd(II) is a distorted square planar
arrangement. The ligand is coordinated to the Pd central atom in
a bidentate mode through the pyridine-N-atom and the N-imine
of the isourea group with the formation of a seven-membered ring
displaying a boat conformation. The two Cl anions are bonded to
the Pd atom in the cis position. Similar coordination modes have
been observed in other PdCl2 complexes [27]. The Pd atom shows
a deviation from the square-planar geometry of 0.0126(2) Å from
the least squares plane defined by the four coordinated atoms. In
the crystal packing the molecules are linked in chains by means
of N3–H. . .Cl2 hydrogen bonds (Table 4). The Pd–N(pyridine) bond
length is 0.02 Å shorter than that between the Pd center and the
coordinating isourea N-atom. The difference between the two
Pd–Cl bond lengths is below 0.01 Å, this indicating similar trans-
influence of the two dents of the N-donor ligand.
Selected geometrical parameters and hydrogen bond parameters for 1Cy (Å and °).
Distances
Angles
Pd1–Cl1
Pd1–Cl2
Pd1–N1
Pd1–N2
C1–O1
C2–O1
C3–N1
C1–N2
C1–N3
C2–C3
2.3001(8)
2.3076(8)
2.026(2)
2.047(2)
1.356(5)
1.387(4)
1.352(5)
1.283(4)
1.319(4)
1.501(5)
Cl1–Pd1–Cl2
Cl1–Pd1–N1
Cl1–Pd1–N2
Cl2–Pd1–N1
Cl2–Pd1–N2
N1–Pd1–N2
C1–O1–C2
Pd1–N1–C3
Pd1–N2–C1
O1–C1–N2
O1–C2–C3
91.49(3)
91.02(7)
175.28(7)
176.62(7)
93.17(7)
84.35(9)
122.2(3)
120.4(2)
119.1(2)
118.6(3)
119.1(3)
Hydrogen
bond
Symm. op. D–H (Å) D. . .A (Å) H. . .A (Å) D–H. . .A (°)
N3–N3. . .Cl2
x, y + 1, z 0.78(4) 3.409(3) 2.66(4) 163(4)
The possible coordination modes of the pyridine–isourea li-
gands towards Pd(II) have been studied also from a computational
point of view, by optimizing the geometries of three possible iso-
mers of 1Pr. In the first isomer the ligand interacts with the metal
center through the pyridine N-atom and the Nimine of the isourea
group, as suggested by the NMR data and shown by the X-ray
structure of 1Cy. In the second isomer the coordination happens
through the pyridine N-atom and the amine nitrogen atom of the
isourea group. Finally, in the last isomer the ligand is bonded to
the metal center through the two nitrogen atoms of the isourea
group and there is no bond between palladium and the pyridine
ring. The first isomer resulted more stable of about 11 kcal molꢀ1
with respect to the second isomer and of more than 20 kcal molꢀ1
with respect to the last isomer considered.
The geometry in solution of the most stable isomer of 1Pr has
been investigated also by means of DFT calculations in combina-
tion with an implicit solvation model for acetone and the resulting
structure is depicted in Fig. 6. The computed Pd—H distance
(2.500 Å) and Pd—H–C angle (103.8°) calculated for 1Pr are typical
for a hydrogen-bonding interaction between a hydrogen atom of
Fig. 6. DFT/C-PCM optimized model for complex 1Pr in acetone.
the methylene group and the metal center. This type of weak bond,
which is typically associated with d8 transition metals centers that
are square planar prior to the interaction, is well known from a
long time [28] and it is actually called ‘‘anagostic’’ [29]. In com-
plexes containing an anagostic bond the interacting hydrogen is
high-frequency shifted, contrary to ‘‘classic’’ agostic interactions.
The great chemical shift difference between the two hydrogen
atoms of the methylene bridge in compounds 1–3 and the presence
of a strong 1H NMR high frequency shift only for one of the two H
atoms could therefore be explained on the basis on such an inter-
action. A partial confirmation of this hypothesis comes from the
Mulliken charge distribution analysis [30]. The computed charges
of the two methylene hydrogen atoms are in fact quite different,
0.216 a.u. for the H atom near the metal center and 0.172 a.u. for
the other one. Also this result agrees with the picture of a weak
pseudo hydrogen bond connecting the metal center and the meth-
ylene fragment, where the bridging H atom is more electron-poor
than the other one. It is to be noted, however, that no trace of inter-
action between the palladium center and the methylene bridge is
present in the X-ray structure of 1Cy depicted in Fig. 5.
4. Conclusions
Fig. 5. ORTEP view of the complex 1Cy showing the thermal ellipsoids at 30%
probability level.
To summarize, in this paper the synthesis of several new palla-
dium(II) complexes with novel pyridine–isourea ligands has been

M. Bortoluzzi et al. / Polyhedron 37 (2012) 66–76
75
Baeza, A. Otero, A. Antiñolo, J. Tejeda, A. Lara-Sánchez, M.I. López-Solera,
Organometallics 26 (2007) 6403.
reported. The characterization data allowed establishing the coor-
dination mode of these ligands, which act as bidentate towards the
Pd(II) metal center, forming a seven-membered ring. This prelimin-
ary work will be prosecuted trying to coordinate these ligands to
other transition metals, to ascertain if the coordination mode de-
pends upon the electronic configuration of the metal center. More-
over, the reactivity of the coordinated pyridine–isourea ligands
will be explored, in particular for what concerns their acid–base
behavior.
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Financial support was provided by the Ca’ Foscari University of
Venice (Ateneo Fund 2010). We acknowledge the CINECA Award
N. HP10CRPVUO, 2011 for the availability of high performance
computing resources and support.
[7] (a) M. Bortoluzzi, G. Paolucci, B. Pitteri, P. Zennaro, V. Bertolasi, J. Organomet.
Chem. 696 (2011) 2565;
Appendix A. Supplementary data
(b) M. Bortoluzzi, G. Paolucci, G. Annibale, B. Pitteri, Inorg. Chem. Commun. 13
(2010) 945;
(c) M. Bortoluzzi, G. Paolucci, B. Pitteri, A. Vavasori, V. Bertolasi,
Organometallics 28 (2009) 3247;
(d) M. Bortoluzzi, G. Paolucci, B. Pitteri, A. Vavasori, Inorg. Chem. Commun. 9
(2006) 1301.
CCDC 853518 and 853519 contain the supplementary crystallo-
graphic data for compound [HpyiuCy]SO3CH3 and complex PdCl2
(pyiuCy) (1Cy), respectively. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/j.poly.
2012.02.014.
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